Optical device incorporating liquid crystals are well known in the art and are used in a variety of applications, including optical phase modulators and variable optical retarders, which are used to impart a variable optical phase delay and/or change the state of polarization of an optical beam. Reflective LC devices may be conveniently fabricated using developed liquid-crystal-on-silicon (LCoS) technology. In a typical LCoS device, a few micrometers (μm) thick layer of an LC fluid is sandwiched between a transparent electrode and a reflective electrode, with the latter formed upon a silicon substrate that also incorporates electrical circuitry required to drive the device. When a voltage is applied to the electrodes, an electric field between the electrodes affects the orientation of LC molecules, which are highly anisotropic. Field-induced orientation of the LC molecules changes an effective index of refraction of the LC layer for an incident optical beam, which affects an optical phase of the optical beam propagating through the LC layer in a way that depends on the polarization of light. When the optical beam incident upon the transparent electrode is linearly polarized either along a predominant direction of orientation of the LC molecules, which is referred to as “director”, or perpendicularly thereto, a variable optical phase delay is imparted to the optical beam by the LC layer of the device without changing the beam polarization, with the value of the phase shift being different for the two orthogonal polarizations. When the incident optical beam is linearly polarized at an acute angle to the LC director, the LC layer can change the polarization state of the optical beam; for example, it can rotate the linear optical polarization, by inducing an optical phase difference between polarization components of the optical beam that are directed along and perpendicular to the LC director.
Arrays of variable optical retarders or phase modulators can be constructed by arranging an array of individually controllable pixel electrodes under a common liquid crystal layer. When a linearly polarized optical beam illuminates such an array, pre-determined optical phase patterns can be imparted to the beam, allowing variable focusing or steering of the optical beam without any moving parts. Arrays of variable optical retarders have found a variety of applications in beam scanning/steering, optical aberrations correction, and so on.
The sensitivity of the LC variable retarders and optical phase modulators to the polarization of the optical beam may be a disadvantage in applications wherein the polarization state of the incident light is not carefully controlled. This sensitivity may be overcome or lessened by incorporating a quarter wave plate between the two electrodes. A quarter wave plate at the reflective electrode of a reflective LC device operates in double-pass as a half wave plate, and switches the polarization state of the light to an orthogonal polarization half-way in its round-trip through the LC layer. In such an arrangement, light that is linearly polarized either along the LC director or orthogonally thereto travels through the LC layer once in its original polarization state, and once in an orthogonal polarization state, resulting at least in theory in a polarization-independent operation.
One drawback of this solution is that the addition of a wave plate between electrodes of an LC device increases the distance between the electrodes, and therefore increases the operating voltage of the device. For example, a typical quartz quarter-wave plate is considerably thicker than a typical LC layer, thus its addition would more than double the required operating voltage. Furthermore, it would be difficult to incorporate such a waveplate into an LCoS device. Form-birefringent (FM) sub-wavelength gratings with a high refractive index contrast may represent a better alternative to conventional waveplates for LC devices. For example, a quarter wave plate for operating at λ=1.55 μm may be realized with an air gaps based FB structure that is of the order of one micron thick. However, these structures may be difficult to fabricate commercially due to their fragility. Alternatively, reflection mode sub-wavelength metallic gratings with a square-wave or similar relief can be employed as waveplates. Although they require a shallower relief depth than FB gratings based on alternating dielectrics, implementing them on the surface of LCOS pixel electrodes may considerably complicate the LCoS fabrication process and reduce device yield.
Accordingly, it may be understood that there may be significant problems and shortcomings associated with current solutions and technologies related to lessening polarization sensitivity of reflective LC devices and to optical wave plates that may be incorporated in such devices.